The present invention relates generally to storage tanks for cryogenic liquids, and more particularly to “double” and “full” containment storage tanks for cryogenic liquids.
Liquefied natural gas (LNG) is transported and frequently stored at temperatures around −261 F (−163 C). Other gases commonly stored in liquid phase below ambient temperatures include ammonia, propane, butane, LPG, ethylene, oxygen, argon, nitrogen, hydrogen, and helium. LNG storage tanks are generally field-erected vessels in sizes of 315,000 to 1,000,000 barrels (50,000 to 200,000 cubic meters).
It is not uncommon for storage tanks for these liquids to have a form of secondary containment. Free-standing tanks often include an inner tank made of stainless steel, aluminum, 9% nickel steel, or other materials suitable for low-temperature or cryogenic service. An outer concrete containment wall can provide secondary containment in the event of a leak in the inner tank. To reduce heat transfer, the inner tank is usually spaced away from the inside surface of the concrete wall, leaving room for thermal insulation. A liquid or vapor barrier on the inside surface of the concrete wall can prevent outside moisture from penetrating the insulation and prevent LNG vapors from escaping to the outside.
Conventionally, the barrier is created using a “paste-on” process or a “stiffened liner” process. In the paste-on process, a thin steel facing is attached to strips of steel that are embedded in the concrete when the concrete wall is poured. In the stiffened liner process, a liner is prepared as part of the inner formwork that the concrete wall is formed against. Internal stiffening is included in the formwork to resist the loads when the wet concrete is poured. After the concrete has set, the internal stiffening is removed, leaving the liner on the inside surface of the concrete.
Conventionally, a roof for these kinds of tanks is constructed in the interior space within the outer wall, and this only begins once the concrete wall has been poured and any formwork or stiffening needed for pouring the wall has been removed from the lower area. Using the conventional processes, significant time is needed between the start of work on the outer wall and the start of work on the roof. In environments where weather can significantly limit the season for outdoor work, shortening this schedule can be beneficial.